The invention relates to a protection device having an evaluation device, which uses instantaneous values of the current and current rate of change to identify a short circuit. Such a device is the subject-matter of EP 0 838 887 A1.
A wide range of implementations of protection devices for low-voltage networks are known from the prior art and are used in particular for disconnection of switching devices when overcurrents occur. By way of example, the measurement device may be in the form of a conventional current transformer. However, such a current transformer normally results in a correspondingly large amount of space being required as a result of the iron circuit that is required and the associated winding space. Nowadays, electronically controlled or microprocessor controlled evaluation circuits, which essentially simulate the function of a bimetallic strip, are generally used for identification of overload situations. Such bimetallic strips are also used in simple devices in conjunction with tripping coils, although these do not offer optimum protection.
In present-day development, switching device design is increasingly demanding space-saving and weight-saving construction, with diagnostic functions and communications capabilities being integrated in the switching device. Thus a need exists for such a device.
The object of the invention is to provide a protection device which, firstly, allows space-saving and mass-saving construction of the protection device while, secondly, allowing rapid identification of anomalous operating states.
The object is achieved according to the invention, in the case of a protection device of the type mentioned initially, by the totality of the features in patent claim 1. Developments are specified in the dependent claims.
In the invention, a unit for early short-circuit identification based on the current and current rate of change is combined with a multiwinding transformer which is based on a printed circuit and once again produces signals which are proportional to the current and current rate of change. The unit for early short-circuit identification can be produced on the basis of known di/dt-i evaluation and, in particular, on the basis of tolerant locus curves. In the case of the latter, so-called TLC methods, locus curves which take account of different power factors are used as tripping criteria in a locus curve representation of the current and current rate of change.
In the invention, means which have already been proposed in another context in the prior art are combined to form a practicable protection device for low-voltage networks. Current transformers which are based on printed circuit boards and are in the form of multiwinding transformers are known, for example, from U.S. Pat. No. 5,321,380 A; while a method for early short-circuit identification which operates especially on the principle of so-called tolerant locus curves (TLC method) is proposed in prior, not previously published German Patent Application 19 729 599 c1.